A DPF is disclosed in Japanese Unexamined Patent Application Publication Laid-open No. Sho 56-129020. Porous honeycomb segments made of ceramic such as silicon carbide are bonded together with bonding layers made of cement to be formed into a honeycomb assembly. This honeycomb assembly is shaped into a predetermined shape such as a circular cross section. Then, the periphery thereof is coated with a coating material layer. This DPF is placed in an exhaust system of a diesel engine to be used for cleaning exhaust gas.
Each of the honeycomb segments is partitioned by porous walls and has a large number of vents extending therethrough in the axial direction. Neighboring vents have one end portions alternately filled. The vents of one type are open at the ends on one side but filled at the ends on the other side. The other vents neighboring to these vents are filled at the ends on the other side but open at ends on the one side.
Exhaust gas flows into the DPF having the above-described structure from the open ends of the vents, passes through the porous walls, and flows out from the other vents. While the exhaust gas is passing through the walls, particulates, notably soot, in the exhaust gas are captured by the walls. Thus, the exhaust gas is cleaned.
In such a DPF, the continuation of exhaust gas cleaning causes soot to be accumulated in the vents. Thus, pressure loss increases with time, and cleaning efficiency decreases. Accordingly, recycling needs to be performed in which the soot is burned and removed. In this recycling, the temperatures of the honeycomb segments are raised by the combustion heat of soot. The temperature distribution during recycling is highest in each central portion of the honeycomb segments on the exhaust gas outlet side (see the publication “SAE Technical Paper Series 870010,” February 1983).
It is assumed that the maximum temperature on the outlet side becomes a temperature of the tolerable temperature or more of such a honeycomb segment during the recycling of the honeycomb segment. In this case, a crack may occur in the honeycomb segment or a catalyst held may be deteriorated.
The following is a known method for limiting the maximum temperature during the above-described recycling. Temperature is controlled so that the inlet temperature of the DPF during recycling is a certain value or less, or the oxygen concentration and flow rate of air supplied during recycling are controlled. A recycling period is adjusted so that the amount of soot accumulated in the honeycomb segment does not become a certain value or more.